I have now received the marriage certificate and the H stands for Herbert, the father is Allen as you suspected and the reason he found himself in Nottingham was because he was a soldier.
I have been trying to find the death of William H Mansell junior born 4/1/1925 unless they have made a mistake when they unredacted him in the 1939. I have found his wife Hazel Victoria nee Stevens death and Probate in 1985 in Buckinghamshire. She has been unredacted as well.
Alan